[codex] Make bm-action sync-specific

[taro-28]

Summary

Make the root basemachina/bm-action action explicitly scoped to bm sync.

Background

The action's existing inputs and behavior are already designed around bm sync: environment selection, --from, dry-run automation, --with-disable, sticky PR comments, and the generated output files are all sync-specific concepts. Keeping a command input would make the public interface look extensible while still exposing options that only make sense for sync, which would become harder to understand as additional BaseMachina CI tasks are introduced.

Since there are no current users to preserve compatibility for, this PR removes the unused command abstraction now and makes the root action's contract explicit.

Changes

• Rename the action metadata to BaseMachina Sync and update the marketplace-facing description.
• Remove the public command input and internal INPUT_COMMAND plumbing.
• Always invoke the CLI with sync as the command.
• Update README and tests to reflect the sync-only contract.

Validation

• bats tests/run.bats tests/auth.bats
• shellcheck scripts/run.sh scripts/auth.sh scripts/_lib.sh
• git diff --check

[github-actions]

mock sync output

[github-actions]

mock sync output

[github-actions]

mock sync output

[github-actions]

mock sync output

[github-actions]

mock sync output

[github-actions]

mock sync output

[github-actions]

mock sync output

[github-actions]

mock sync output